Echo profile probe

ABSTRACT

In an orthogonal frequency division multiplex (OFDM) communication network, a node transmits an echo profile probe to other nodes in the network. The echo profile probe is a message that allows characterization of the unique echo profile through the communication channel between each node pair. The echo profile is used to calculate the cyclic prefix length needed for optimum communication from one node to the other.

RELATED APPLICATIONS

This application claims priority from U.S. provisional patentapplication Ser. No. 60/633,257 filed Dec. 3, 2004 entitled “EchoProfile Probe”, incorporated herein by reference.

BACKGROUND

1. Field of Invention

The invention relates to broadband communication networks.

2. Prior Art

Orthogonal Frequency Division Multiplexing (OFDM) is a modulation schemeused in communication systems. It is a technique used in multi-antennabroadband systems since it significantly reduces the complexity of thereceiver by providing orthogonal sub-channels.

Multipath effects in OFDM are greatly reduced by adding a cyclic prefix(CP) to each OFDM symbol. The CP acts as a guard interval betweensuccessive OFDM symbols. In conventional OFDM systems, a fixed-length CPis used. If the channel delay spread is less than or equal to the CP,inter-symbol interference (ISI) is prevented. When the receiver ismoving from one environment to another, this condition may not be met,ISI will occur, and the system will be impaired.

The phase noise in OFDM can cause inter-channel-interference (ICI). Theuse of a cyclic prefix also reduces ICI.

A drawback to OFDM and the use of a fixed-length CP is the reduction inrate due to the CP overhead. If the fixed length CP is not long enough,system performance deteriorates. If the CP length is too long, spectrumefficiency is reduced.

Zhang et al., “A novel OFDM transmission scheme with length-adaptiveCyclic Prefix”, J Zhejiang University SCI, 2004, describes a techniqueapplied to a mobile wireless environment to create a variable length CPin order to counter the aforementioned drawbacks of fixed CP length. Thepaper describes a system where the CP length is reduced when delayspread is small thus reducing overhead. Conversely, the CP length isincreased when delay spread is high and ISI eliminated. The disclosedtechnique utilizes known symbols in the preamble of each packettransmission or pilot sub-carriers to estimate the channel parameters.The channel parameters are used to determine the RMS delay spread andthe resulting CP length needed. The technique is inefficient in slowlychanging environments because of the preamble overhead or the pilotsymbols present in each transmission. Accuracy in the determination ofthe channel parameters decreases as the number of pilot symbols isreduced to increase spectrum efficiency. In order to compute the delayspread accurately, a long sequence is needed, but the continualtransmission of a long sequence uses bandwidth inefficiently.

SUMMARY OF THE INVENTION

The present invention uses an echo profile probe as part of a techniqueof determining the channel impulse response (CIR) of a communicationchannel in order to set appropriate communication parameters includingcyclic prefix (CP) length for orthogonal frequency division multiplex(OFDM) communication. Without probing the channel, the CP length wouldhave to be set conservatively and long. The echo profile probe is aknown message transmitted by any node to other nodes. The receivingnodes, having a priori knowledge of the probe message, perform acomputation on the received message, and determine delay spread and therequired CP length to avoid inter-symbol interference (ISI).

When a new node enters a network, the nodes need to determine channelcharacteristics with each other node. The new node broadcasts the echoprofile sequence to each node, and each receiving node returns itscomputation to the new node. Each node also sends an echo profile to thenew node for computation and response. The computations include thedetermination of the minimum cyclic prefix needed. By minimizing the CPlength for each packet transmission, overhead due to the CP is reducedand the bandwidth efficiency and performance of the system is optimized.

The present invention uses a separate message, a probe message, todetermine the channel characteristics and thereby determine the optimumCP length, which overcomes a limitation of other adaptive schemes thatuse constantly transmitted symbols in each packet to determine thechannel characteristics. The use of the probe or unique message of thisinvention allows more symbols to be used for channel characterization,providing increased accuracy in calculating delay spread and required CPlength. Because the cable environment is considered nearly static, theprobe is sent infrequently thus reducing the overhead required foroptimum channel characterization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Echo Profile Probe Structure

FIG. 2 Illustration of Delay Spread

FIG. 3 New Node Entering Network

FIG. 4 Details of Communication between New Node and Existing Node

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the echo profile probe is a message 100 used todetermine the channel impulse response. The message 100 consists of apreamble 101 and a time-domain data payload 104. The preamble containsinformation for gain control 102, and for timing and frequency-offsetadjustments 103. The payload contains the echo profile sequenceconsisting of 1024 pseudo-random BPSK-modulated bits. This sequence isgenerated using an m-sequence of degree 10, given by the generatorpolynomial 2201₈ with an initial seed of 3FF₁₆.

In the receiver, the received Echo Profile Sequence (rx) iscross-correlated to a local copy of the same sequence (lseq) as shown inEquation 1.

$\begin{matrix}{{corr\_ out} = {\sum\limits_{i = 0}^{1024}\;{{{rx}\left( {t - i} \right)}*{{lseq}(i)}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Referring to FIG. 2, the time distance between the earliest significantcorrelation peak 200 and the latest significant correlation peak 201 isthe significant delay spread in the channel response. This delay spreadcan be used to determine the minimum cyclic prefix that would maintainthe orthogonality of the OFDM symbol and eliminate ISI.

The time distance between peaks is determined by further processing ofthe output of the correlation (corr_out from Equation 1). An example ofthe processing steps are described below:

-   -   Step 1: The peak value of corr_out is determined. This is found        using the following pseudo-code and recorded as MAX_PEAK. The        time where the MAX_PEAK occurs is recorded as PEAK_TIME.        -   IF (MAX_PEAK>corr_out) MAX_PEAK=corr_out    -   Step 2: Look for the first instance of a peak that exceeds a        threshold X dB below the peak and record this time as        START_PROFILE. X assumes an initial value determined by system        analysis for a particular system and is a function of the echo        amplitude expected in the channel.    -   Step 3: Look for the last instance of a peak that exceeds a        threshold X dB below the peak and record this time as        END_PROFILE.    -   Step 4: Calculate CP_LEN=END_PROFILE_START_PROFILE.    -   Step 5: If CP_LEN is greater than CYC_PREFIX_MAX, reduce X by 1        dB and repeat step 2-5 until CP_LEN is smaller than        CYC_PREFIX_MAX. CYC_PREFIX_MAX is the maximum cyclic prefix        allowed in the data path and equals to 64 in the current        embodiment.    -   Step 6: Reduce X by 1 dB and repeat step 2, 3 and 4. If the        CP_LEN is less than R*CYC_PREFIX_MAX (where R is typically a        value between 0 and 1) use the newly calculated CP_LEN.        Otherwise, use the previously calculated CP_LEN. In this        embodiment, step 6 is only done once, but step 6 may also be        applied repeatedly with increasingly smaller R until CP_LEN is        greater than R*CYC_PREFIX_MAX    -   Step 7: Compute SYSTEM_BIAS_TIME as PEAK_TIME−START_PROFILE.    -   Step 8: Increase the CP_LEN such that CP_LEN=CP_LEN+DSG_MARGIN.        DSG_MARGIN is dependent on implementation, for example 25.

FIG. 3 illustrates the used of the echo profile probe when a new node300 enters a network with existing nodes 301, 302, and 303. New node 300broadcasts its echo profile probe to each of the existing nodes. Each ofthe existing nodes returns results of computations to node 300.

FIG. 4 illustrates the communication between new node 400 and existingnode 401. The new node 400 initially transmits its echo profile probe tothe existing node 401. The existing node 401 receives the message andperforms the necessary computations. The existing node 401 thentransmits the calculated cyclic prefix length to the new node 400followed by the transmission of its echo profile probe. The new node 400receives the probe and performs the needed calculations, and then sendsback to the existing node 401 its calculated cyclic prefix length.Existing node 401 transmits an echo profile probe to new node 400, whichperforms computations and transmits a cyclic prefix length to existingnode 401. The probe message can be sent multiple times and the resultsaveraged for increased accuracy.

In one embodiment of the invention, a cycle master, also called NetworkCoordinator (NC), controls admission of a new node. A map broadcast bythe NC schedules echo profile probe slots along with all other messages.Existing nodes expect a probe message at the slot time designated in themap.

The present invention is suitable for use in a network operating over acommunication channel formed by interconnecting nodes with coaxial cableand passive signal splitters.

1. A method of communicating using orthogonal frequency divisionmultiplex (OFDM) signaling between a first node and a second node in acommunication network comprising the steps of: sending an echo profileprobe from the first node; receiving the echo profile probe at thesecond node; calculating a cyclic prefix length from the received probemessage contents; transmitting the cyclic prefix length from the secondnode to the first node; and sending messages from the first node to thesecond node using the cyclic prefix length; wherein the communicationnetwork comprises nodes interconnected with coaxial cable and passivesignal splitters; and wherein the cyclic prefix length is calculated bydetermining the channel delay spread wherein echo peaks are identifiedby cross-correlation between the received echo profile probe and thesent echo profile probe, the channel delay spread is calculated as thedelay between echo peaks, then a predetermined design margin value isadded to the channel delay spread to produce the cyclic prefix length.2. The method of claim 1 wherein the echo profile probe is sentaccording to a map broadcast by a network controller.
 3. A method ofcommunicating using orthogonal frequency division multiplex (OFDM)signaling between a first node and a second node in a communicationnetwork comprising the steps of: sending an echo profile probe from thefirst node; receiving the echo profile probe at the second node;calculating a cyclic prefix length from the received probe messagecontents; transmitting the cyclic prefix length from the second node tothe first node; and sending messages from the first node to the secondnode using the cyclic prefix length; wherein the echo profile probecomprises 1024 pseudo-random BPSK-modulated bits generated using anm-sequence of degree 10 given by the generator polynomial 2201₈ with aninitial seed of 3FF₁₆.